1. Technical Field
This invention relates to an improved process for preparing BaFBr:Eu phosphors for use in x-ray intensifying screens, as well as the phosphors made by the process.
2. Background Art
The development and use of x-ray intensifying screens based on barium halides such as BaFCl:Eu permitted a significant decrease in patient exposure to x-rays because BaFCl:Eu has the ability to absorb a larger fraction of the incident x-rays and convert this absorbed x-ray radiation to a greater amount of visible light than the widely used screens based on calcium tungstate. The speed of BaFCl:Eu based screen systems is generally at least twice the speed of calcium tungstate based screen systems.
BaFBr:Eu has similar chemistry to BaFCl:Eu but has a higher physical density, (D=4.96 g/cm.sup.3 for BaFBr versus D=4.51 g/cm.sup.3 for BaFCl) and, therefore, BaFBr:Eu is able to absorb a larger fraction of incident s-rays per unit volume, thus allowing a further reduction in x-ray dosage.
A. L. Stevels and F. Pingault, Phillips Research Report, Vol. 30 #5, pp 277-290, 1985, describe attempts to prepare BaFBr:Eu using conventional solid state techniques employing dry mixing and firing, followed by milling of the product to the desired particle size. These attempts failed, yielding phosphors with too high an afterglow (lag) for medical applications.
U.S. Pat. No. 4,076,897 (1978) discloses the reduction of lag in BaFBr:Eu phosphors by the inclusion of potassium or rubidium ions, typically added as their halide, hydroxide, carbonate or nitrate salts.
U.S. Pat. No. 4,157,981 (1979) discloses a process for preparing BaFBr:Eu that comprises forming a suspension of BaF.sub.2 in an aqueous solution of BaBr.sub.2, which solution contains an equimolar amount of BaBr.sub.2 in dissolved form per mole of BaF.sub.2, evaporating said suspension to dryness at a temperature of 50.degree. to 250.degree. C., mixing the resultant BaFBr with an appropriate amount of EuBr.sub.3, heating the resultant dried mixture in a weakly reducing atmosphere of nitrogen and a small amount of hydrogen for effectively reducing trivalent europium to bivalent europium, at a temperature of 600.degree. C. to 1000.degree. C., and then, after cooling, heating the resulting BaFBr:Eu, in an inert atmosphere, at a temperature of 600.degree. C. to 850.degree. C. BaFBr:Eu prepared by this method is deficient with respect to lag compared to both calcium tungstate and BaFCl:Eu.
U.S. Pat. No. 4,532,071 (1985) discloses a process for preparing BaFBr:Eu phosphors doped with various metal oxides, e.g., SiO.sub.2, Al.sub.2 O.sub.3, MgO, CaO, etc., said phosphors said to be usable in either x-ray intensifying screens or in radiation image storage panels. It is stated that lag is reduced in x-ray intensification applications, but no data are given. The purpose of the oxide is to reduce or prevent sintering of the phosphor during the second of two firing steps. It is also stated that various phosphors, including BaFBr, are not prepared by spray drying a suspension of BaF.sub.2 and, for example, BaBr.sub.2.
U.S. Pat. No. 4,524,016 (1985) discloses a process for the preparation of spheroidal particulate BaFCl:Eu phosphor having a narrow size distribution which process comprises: (a) spray drying a slurry comprised of BaFCl, or the precursors thereof, a europium halide, and a BaCl.sub.2 flux in a liquid under conditions which produce porous spheroidal agglomerates of BaFCl, or the precursors thereof, BaCl.sub.2, and the europium halide, the agglomerates having an average diameter larger than the desired diameter of the particulate phosphor; (b) firing the porous spheroidal agglomerates of step (a) in a flowing inert atmosphere for about 10 to about 120 minutes at a temperature of about 650.degree. C. to about 950.degree. C.; (c) cooling the fired material of step (b) to ambient temperature in an inert atmosphere; (d) washing the cooled material of step (c) with water to remove BaCl.sub.2 ; and (e) drying the washed material of step (d) to remove the water and to produce a free flowing powder consisting essentially of spheroidal particulate BaFCl:Eu phosphor.
Solid state preparation techniques for BaFBr:Eu phosphors that employ milling or grinding of the product near or at the final stage result in the production of undesirable subsize fine particles and also result in damage to the surface of phosphor particles which is believed to contribute to afterglow (lag) by generating surface traps. In addition to increased afterglow, a consequence of this less than optimum particle morphology on the performance of the phosphor is a lower than optimum light output due to light scattering by generated fines.
The purpose of this invention is to prepare high speed BaFBr:Eu phosphor having very low afterglow, this latter being aided by the avoidance of the grinding or milling of the phosphor. A further goal is to prepare said high speed phosphor while avoiding costly and potentially hazardous firing steps that involve the use of hydrogen.
Phosphor preparation techniques that do not employ milling or grinding generally call for the use of water soluble fluxes which are washed out with water after the firing step. Examples of this are the use of BaCl.sub.2 flux for BaFCl, phosphor (U.S. Pat. No. 4,524,016) and Li.sub.2 SO.sub.4 flux for the preparation of LnTaO.sub.4 phosphors (U.S. Pat. No. 4,225,653). These techniques are not applicable to the preparation of BaFBr because BaFBr is unstable in the presence of pure water where it decomposes via hydrolysis.
It has been found that, in spite of the tendency for BaFBr to decompose in the presence of water, this material, as well as its europium doped analog, can be formed in and precipitated from aqueous media using the present invention. Non-aqueous media can also be employed.
By employment in this process, it is possible to prepare BaFBr:Eu phosphor without any harmful grinding and milling operations. As a consequence of this, the resulting phosphor has a higher light output and a lower lag than reported for BaFBr:Eu prepared by any other method.